Recently, many countries have tried to develop wind power, tidal power, geothermal energy, solar energy, hydrogen gas, etc. as energy sources for replacing depletable fossil fuels. Especially, from among such energy sources, hydrogen gas has the highest energy efficiency per unit mass and no harmful byproducts of combustion, and thus research on the preparation, storage, transportation, etc. thereof has been conducted. In particular, a focus is placed on research on the practical use of a fuel cell and the development of a material for efficiently storing hydrogen gas.
Currently, materials capable of storing hydrogen gas include metal hydride, carbon nanotube, carbon compound such as activated carbon, zeolite, metal-organic framework (MOF), etc. Especially, the MOF has been noticed due to a higher specific surface area than those of other materials, and accordingly, the possibility of reversibly storing hydrogen.
The MOF is a kind of organic-inorganic hybrid compound, in which a metal and an organic ligand are three-dimensionally linked via the organic ligand functioning as a linker. Specifically, as shown in FIG. 1, the MOF refers to a material in which the organic ligand is coordinated to at least two metals, and each of the coordinated metals is coordinated in a chain-like manner to at least one other organic ligand, thereby forming many tiny spaces, i.e. a network structure with pores, inside the framework.
Such a MOF is prepared by various preparation methods. For example, the MOF can be prepared through a substitution reaction of organic ligand ions by using metal salts as a metal source. Specifically, in such preparation, zinc nitrate [Zn(NO3)2] as the metal source, and a dicarboxylic acid-based compound as the ligand are mainly used so as to prepare the framework (O. M. Yaghi et al. Science, 2003, vol. 300, p. 1127; WO 02/088148).
Also, an isoreticular metal-organic framework (IRMOF) can be prepared by using zinc as the metal source to thereby form core zinc oxide (Zn4O) and by using an organic ligand such as a dicarboxylic group. In addition, metal ions such as Cu and Fe (instead of zinc) as a core, and a tridentate or multidentate organic ligand can be used to prepare a MOF.
As described above, in a prior art, various organic ligands and metals have been used to prepare MOFs having various structures in such a manner that a MOF can store as much hydrogen as possible. However, it has been known that such a conventional MOF cannot store a large amount of hydrogen gas at an ambient temperature and an atmospheric pressure, so the storage capacity of the hydrogen gas does not reach a required level. In other words, in the conventional MOF, hydrogen gas is adsorbed to only some spaces, from among the whole spaces for adsorbing hydrogen gas, and most of the spaces remain empty. Thus, the storage of hydrogen gas is not efficient.
In addition, when an ambient pressure or temperature changes, the conventional MOF reversibly physically adsorbing hydrogen gas can not keep its stable storage of the hydrogen gas. Accordingly, in storing and/or transporting the hydrogen gas by using the MOF, expensive equipment has been required to maintain a fixed temperature or pressure. Therefore,
Matthew J. Rosseinsky et al. provided Ni2(bipy)3(NO3)4, that is, a MOF which can irreversibly physically adsorb hydrogen, in “Hysteretic Adsorption and Desorption of Hydrogen by Nanoporous Metal-Organic Frameworks”(Science, Vol. 306, p. 1012). However, this MOF has a two-dimensional structure (like a structure of graphite), not a three-dimensional structure, and thus is less suitable for a material for storing hydrogen due to instability in the structure (for example, the structure may be easily broken).